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Commit b2c4623d authored by Paul E. McKenney's avatar Paul E. McKenney
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rcu: More on deadlock between CPU hotplug and expedited grace periods 



Commit dd56af42 (rcu: Eliminate deadlock between CPU hotplug and
expedited grace periods) was incomplete.  Although it did eliminate
deadlocks involving synchronize_sched_expedited()'s acquisition of
cpu_hotplug.lock via get_online_cpus(), it did nothing about the similar
deadlock involving acquisition of this same lock via put_online_cpus().
This deadlock became apparent with testing involving hibernation.

This commit therefore changes put_online_cpus() acquisition of this lock
to be conditional, and increments a new cpu_hotplug.puts_pending field
in case of acquisition failure.  Then cpu_hotplug_begin() checks for this
new field being non-zero, and applies any changes to cpu_hotplug.refcount.

Reported-by: default avatarJiri Kosina <jkosina@suse.cz>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: default avatarJiri Kosina <jkosina@suse.cz>
Tested-by: default avatarBorislav Petkov <bp@suse.de>
parent f114040e

kernel/cpu.c

+13 −1
Original line number Original line Diff line number Diff line
@@ -64,6 +64,8 @@ static struct { 
	 * an ongoing cpu hotplug operation.

	 * an ongoing cpu hotplug operation.

	 */

	 */

	int refcount;

	int refcount;

	/* And allows lockless put_online_cpus(). */

	atomic_t puts_pending;





#ifdef CONFIG_DEBUG_LOCK_ALLOC

#ifdef CONFIG_DEBUG_LOCK_ALLOC

	struct lockdep_map dep_map;

	struct lockdep_map dep_map;
@@ -113,7 +115,11 @@ void put_online_cpus(void) 
{

{

	if (cpu_hotplug.active_writer == current)

	if (cpu_hotplug.active_writer == current)

		return;

		return;

	mutex_lock(&cpu_hotplug.lock);

	if (!mutex_trylock(&cpu_hotplug.lock)) {

		atomic_inc(&cpu_hotplug.puts_pending);

		cpuhp_lock_release();

		return;

	}





	if (WARN_ON(!cpu_hotplug.refcount))

	if (WARN_ON(!cpu_hotplug.refcount))

		cpu_hotplug.refcount++; /* try to fix things up */

		cpu_hotplug.refcount++; /* try to fix things up */
@@ -155,6 +161,12 @@ void cpu_hotplug_begin(void) 
	cpuhp_lock_acquire();

	cpuhp_lock_acquire();

	for (;;) {

	for (;;) {

		mutex_lock(&cpu_hotplug.lock);

		mutex_lock(&cpu_hotplug.lock);

		if (atomic_read(&cpu_hotplug.puts_pending)) {

			int delta;



			delta = atomic_xchg(&cpu_hotplug.puts_pending, 0);

			cpu_hotplug.refcount -= delta;

		}

		if (likely(!cpu_hotplug.refcount))

		if (likely(!cpu_hotplug.refcount))

			break;

			break;

		__set_current_state(TASK_UNINTERRUPTIBLE);

		__set_current_state(TASK_UNINTERRUPTIBLE);